The invention relates to a method for generating wide-band seismograms while simultaneously suppressing correlated noise signals.
This method can be applied advantageously but not exclusively to seismic exploration in which the seismic energy radiated into the ground is generated by means of one or more vibrators. As a rule these vibrators allow a signal to be transmitted with controlled pre-defined frequency content. As with all other energy sources which act on the surface of the earth a substantial proportion of the generated seismic energy does not penetrate into the ground but is propagated along or near the surface, for example in the form of surface waves or ground-coupled air waves and in the form of shallow refracted waves. Owing to their slow amplitude reduction these noise waves, which are causally associated with signal generation and are therefore called correlated noise signals, usually have amplitudes which are greater than the body waves, hereinafter referred to as desired signals, which penetrate into the ground and are reflected to the earth's surface from interfaces of formations with different elastic properties. It is common practice to install a plurality of geophones, also denoted as receivers, in one line in the profile direction, i.e. in the direction from the energy source to the receivers, so that the ground motion recorded by the geophone is not controlled exclusively by the waves which are propagated along the earth's surface or at least close to the earth's surface. The use of so-called transmitter groups, i.e. a plurality of signal sources, installed in the profile direction and adapted to operate simultaneously or sequentially, hereinafter also referred to as transmitters, also serves to suppress the surface-coupled noise signals apart from increasing the signal energy relative to the ambient noise.
In this context serial configurations in the profile direction are equivalent to area configurations which additionally suppress noise signals which arrive on the receiver side from the side and are usually unrelated to signal generation. The arrangement of transmitters and receivers in a line works as a wavelength filter and to a high degree attenuates waves whose apparent wavelength is of the same order of magnitude or smaller than the length of the transmitter and/or receiver group. For this reason transmitter and receiver groups will be referred to hereinbelow also as wavelength filters. The method of operation of receiver and transmitter groups and the calculation of their attenuation curves have been described in detail in the relevant literature (inter alia Th. Krey, F. Toth; Remarks on Wavenumber Filtering in the Field, Geophysics, 38, pages 959-970, 1973).